I can only speak for the Ilford Multigrade filter set, but I've never had an instance where the base exposure (for high values) changed when moving to a different grade. I should be clear here that I don't use the extreme ends of the filter range so, perhaps, that's why I don't see any significant change. That said, I always keep all my test strips and test prints available in a water tray until I reach an acceptable work print. Once I have the work print, the base exposure, contrast grade, and development have pretty much been finalized. At this point, I move on to refining the work print working toward that elusive creature...the fine print.Once you have established your time based on the high values, what happens to the high values when increasing the filter grade to darken the low values.
I didn't mention in my first post, but after the initial test strip trials (I do 3 sec strips) many times I can't really determine if, say, the 4th or 5th strip is what I want so I make another "test strip" using a full size sheet of paper starting with four 3 sec exposures (total of 12 secs), then resetting the timer for 1 sec exposures, and continuing with strips up to 15 secs. At this point, I have a full sheet of test strips with exposures of 12, 13, 14, and 15 secs; 1 sec difference in exposure can result in a visible difference in your high print values. Occasionally, I even narrow things down to 1/2 sec exposures! But, I'm crazy!!
Ian G - I've always advocated that you should be kind to the mole in the darkroom when out working in the field!